Hypothalamic gonadotrophin releasing hormone (GnRH, also known as luteinizing hormone releasing hormone, LHRH) mediates the regulation of pituitary gonadotrophin synthesis and secretion. Since the initial isolation of LHRH by Schally et al. (1971) numerous analogues have been synthesized, which may be classified according to their acute effects on gonadotrophin release, as either agonists (with enhancement of release) or antagonists, (with inhibition of release). The agonists are characterised by an increase in binding affinity to pituitary LHRH receptors (Loumay et al, 1982, Perrin et al. 1980) and by increased resistance to the proteolytic degradation that rapidly removes native LHRH.
The observation that these potent analogues can induce potentially reversible medical castration has provided a new approach to the treatment of various gonadotrophin dependent disorders, particularly hormone dependent prostate and breast cancer. Two distinct phases in the induction of castration occur. Initially the analogue stimulates the pituitary-gonadal axis causing a transient increase in gonadotrophin and sex steroid secretion in the first two weeks or so. After this period there is a down regulation of pituitary LHRH receptors with subsequent decline in gonadotrophin and sex steroid secretion. The initial stimulation is a substantial drawback to the use of agonists in the treatment of prostatic malignancy as the initial stimulation of testosterone can produce a painful flare of the disease with consequent adverse clinical effects (Eisenberger and Abrams, 1988, Crawford and Davis, 1988). By contrast the first successful synthesis of LHRH antagonists should be advantageous in avoiding the initial flare. First attempts at the production of antagonists led to compounds characterised by undesirable histamine release (Karten and Rivier, 1986, Schmidt et al., 1984, Phillips et al ., 1988). However new antagonists have been developed characterised by improved potency and much less histamine releasing potential eg. Karten and Rivier, 1986. One of the most potent antagonists described to date is the analogue N-Ac-D-Nal(2), D-Phe (pCl), D-Pal(3), Ser, Lys (Nic), D-Lys(Nic), Leu, Lys(iPr), Pro, D-Ala-NH2 (ANTIDE), synthesized by Folkers and Bowers. It is highly potent in rats in inhibiting ovulation (Ljundquist et al;, 1988) and single doses have profound, long lasting inhibitory effects on serum LH concentrations in castrate female cynamologous monkeys (Leal et al, 1988). It has also been shown to be capable of inducing long term chemical castration in intact adult male rats and cynamologous monkeys, and to have an inhibitory effect on tumour growth in the Dunning R3327 prostatic carcinoma model, similar to that of castration (Habenicht et al, 1990). The new antagonist should therefore be of substantial clinical interest in all those conditions in which medical castration is desirable, particularly in the management of prostatic cancer and in various gynaecological disorders (McLachlan et al., 1986). Current administration is limited to the parenteral route. However the dose of analogue that can be delivered by this route is limited due to the poor solubility of the antagonist. Thus clinical trials of the long term effect of ANTIDE have been reduced to doses of 2.5 mg or lower.
The oral route of administration of peptides such as LHRH and its analogues as pharmaceuticals in the treatment of systemic conditions has met with little success. In general the amount of peptide required for successful oral administration has been 100 to 1000 times the dose required for parenteral deliver, thus making the administration of these agents via this route prohibitively expensive. There are two fundamental reasons for the lack of success. Firstly, the intestinal milieu has a high degree of proteolytic activity, which rapidly degrades most peptides. Secondly, while there are well defined uptake mechanisms for individual amino acids and di-peptides, there is no general mechanism for polypeptides to be transported across the membrane of the mucosal epithelium into the circulation. Rather, this membrane is designed as a general barrier prohibiting the uptake of the numerous foreign proteins encountered in this environment. Thus, although a peptide may be modified to withstand the enzymatic barrage encountered in the intestine, such modification is of little value if the peptide cannot subsequently cross the mucosal barrier and enter the systemic circulation.
Recent work by the current inventor (PCT/AU86/00299) has however provided a method to overcome the mucosal barrier. This method takes advantage of the natural intrinsic factor (IF) mediated uptake mechanism for vitamin B12 (VB12). VB12 is a naturally occurring dietary molecule which is actively taken up from the intestine. During this process it first binds to IF in the upper small intestine. The VB12-IF complex then passes down the small intestine and binds to an IF receptor located on the surface of the ileal epithelium. The whole VB12-IF-Receptor! complex is then internalized by receptor-mediated endocytosis and some time later the VB12 appears in serum.
PCT application PCT/AU86/00299 (WO87/02251) describes methods to modify chemically VB12 to provide suitable functional groups for conjugation of the VB12 to various drugs and peptide/protein pharmaceuticals. When the VB12-pharmaceutical! complex is administered orally it is possible to utilise the natural IF-mediated VB12-uptake system to deliver the pharmaceutical to the circulation.
As the oral route of administration is the most preferable means of delivering a pharmaceutically active agent, there is a large and valuable market for any process which permits the oral delivery of LHRH antagonists to humans. Such a process would be available by the formation of a complex between VB12 or analogues thereof and LHRH antagonists.